Precision linear motion converting and position measuring apparatus

ABSTRACT

A roller-band device including an idler roller, drive roller and band, is mounted on a carriage movable linearly relative to the band so that the angle of rotation of the drive roller is related to the distance traveled by the carriage. The drive roller can be driven to convert rotary motion to linear motion or the carriage can be driven to convert linear motion to rotary motion. Means including a pulse generator coupled to the drive roller produces a very accurate readout of the position of the carriage.

[ Oct. 24, 1972 United States Patent Mermelstein [541 PRECISION LINEARMOTION 3,264,739 8/1966 Berlinsky et al. ............33/123 CONVERTINGAND POSITION 3,467,900 9/1969 Benton......................318/592MEASURING APPARATUS 3,571,932 3/1971Peddie.........................33/123 [72] Inventor: Seymour Memelstein,Newton 396,430 1/1889 Re1chel.....................74/89.22

Mass.

[73] Assignee: Tyeo Laboratories, Inc., Waltham,

Primary Examiner-William F, ODea Assistant ExaminerWesley S. Ratliff,Jr. AttorneySchiller & Pandiscio Mass.

[57] ABSTRACT A roller-band device including an idler roller, drive [22]Filed: Aug. 6, 1970 [2]] Appl. N0.: 61,725

roller and band, is mounted on a carriage movable [52 us. 74/8922linearly relative to the band that the angle of [51] Int. 27/02 thedrive is related the distan 58] Field of Search...........74/25, 89.2,8921,8922; traveled by camage- The drive mile n be 318/592; 250/211,231; 33/123 driven to convert rotary motionto linear motion or thecarriage can be driven to convert linear motion to References Citedrotary motion. Means including a pulse generator cou- UNITED STATESPATENTS pled to the drive roller produces a very accurate readout of theposition of the carriage.

3,210,632 Benton et al. 18/592 14 Claims, 6 Drawing Figures PATENTEI]OCT 24 I972 SHEET 2 OF 3 Pig. 5. I H

I NVEAJ OR SEYMOUR MERMELSTE/N BY g7 pamlijcio ATTORNEYS PATENTED 24I97? 3 6 99,819

SHEET 3 OF 3 This application is concerned with a linear-rotary motionconverter incorporating a roller-band device providing very accuratecorrelation between the linear motion of one component of the device andthe rotary motion of another component. This application is particularlyconcerned with apparatus incorporating the roller-band device forproducing a very accurate measurement of the distance traveled by thelinearly movable component.

Roller-band drives useful, for example, as suspension systems, aredisclosed in US. Pat. No. 3,471,668 dated Oct. 7, 1969, as including aroller-band cluster comprising a pair of aligned drive rollers and aresilient band having an S-shaped configuration convoluted around andbetween the rollers, and a guideway having a rectilinear cross sectionproviding four planar surfaces for containing the roller-band cluster.

The primary objects of the invention are: to provide a novel andimproved roller-band device particularly adapted for converting linearmotion of the roller-band cluster into rotary motion of one of therollers and vice versa; and to provide a device as described exhibitingextremely accurate correlation between linear travel of the rollersrelative to the band and the angle of rotation of one of the rollers.

Motion converting devices of the type described and find utility in avariety of application in which a component of an apparatus is requiredto be moved linearly through a precise distance to an accuratelydetermined position. Typical examples of such applications includemeasuring devices and machine tools. Rotary transducers are known formeasuring angle of rotation with extreme accuracy, for example, i 1minute of arc. An example of an accurate, yet relatively inexpensivetransducer for measuring angular position is a rotary pulse generator ofthe type having an output of thousands of electrical pulses perrevolution, coupled with a pulse counter. With apparatus comprising theimproved roller-band device of the invention in combination with arotary pulse generator, it has been possible to establish the positionof a linear movable member, such as the table of a milling machine, witha resolution of 0.0001 inch and an accuracy of 1 0.0001 inch. Moreoverit has been possible to achieve similar accuracy over a wide range ofdistances and table positions, for example, 12 inches.

Thus with such an angular measuring device coupled with one of therollers of applicant's improved rollerband cluster providing precisecorrelation between linear and rotary motion, it is possible to achieveanother object of the invention, namely, the provision of a relativelyinexpensive, yet extremely accurate, linear measuring apparatus.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

The invention accordingly comprises the apparatus possessing theconstruction, combination of elements and arrangement of parts,which-are exemplified in the following detailed disclosure, and thescope of the application of which will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description taken inconnection with the accompanying drawings wherein:

FIG. 1 is a perspective view, partially in section, illustrating thelinear measuring apparatus coupled with the table of a milling machineto indicate the position of the table;

FIG. 2 is a side elevational view ofthe linear measur ing apparatus ofFIG. 1;

FIG. 3 is a front elevational view of the improved roller-band device ofthe invention;

FIG. 4'is a perspective view, partially in section, of the device ofFIG. 3; v v

FIG. 5 is a sectional view taken substantially along the line 55 of FIG.3; and

FIG. 6 illustrates a modification of the invention.

As previously noted, the improved roller-band device of the inventionfinds utility in measuring apparatus and particularly in machine toolsin which it is necessary to accurately locate a work piece with respectto a tool. A typical machine tool in which the invention can beincorporated to advantage is a conventional milling machine such asillustrated in FIGS. 1 and 2, including a horizontal table 10 movablelinearly with respect to a motor-driven spindle 12 on which a rotarycutting tool is mounted. The table is designed to support and move aworkpiece linearly relative to the tool mounted on the stationaryspindle. The length .of travel of the table which supports the workpiecemay. be relatively long, i.e., several feet, and the apparatus of theinvention is designed to accurately position the table throughout itsentire travel distance.

A basic component of the linear measuring apparatus of the invention isa roller-band device, generally designated 14. As shown in detail inFIGS. 3 through'5, the roller-band device includes a roller-band clustercomprising a cylindrical idler roller 16, a cylindrical drive roller 18and an elongated, resilient (flexible) band 20 including an intermediateportion convoluted in an S-shaped configuration around and between theidler and drive rollers. The rollers 16 and 18 are mounted on one of thetwo relatively movable (linearly) components of the apparatus, in thiscase, the base 22 of the milling machine on which the table is slidablymounted, and the band 20 is mounted on the other component, namely, thetable 10 of the milling machine, so that the rollers move linearlyrelative to the band between which they are engaged. Clamps 24 mountedon table 10 are provided for securing the opposite ends of band 20 andholding the band 20 in tension. Two bellows 26 supported on a wire 28having its ends attached to clamps 24 are provided for protectivelyenclosing band 20. Each of the bellows 26 (only one of the bellows isshown in FIG. I for convenience of illustration) is attached at one endto a clamp 24 and at its other end to a housing 30 protectivelyenclosing the roller-band cluster and other components of the apparatus.The housing 30 is attached to the milling machine base by cap screws asshown at 31in FIG. 2.

The improved roller-band device of the invention is especially adaptedto establish a very accurate correlation between linear motion ofrollers 16 and 18 relative to band 20 and the angle of rotation of driveroller 18. This requires a construction designed to eliminate errors dueto friction, stretching of the band, temperature changes, variations incurvature and tangency of the band, and twisting, skewing, lateralmovement and camber of the band. The band itself is formed of a thinflexible inelastic material such as steel, beryllium copper or the like,having a uniform thickness and width. All of the rollers which contactthe band are cylindrical and except for idler roller 16, are journalledin precision bearings. All of the rollers are fabricated within closeconcentricity and eccentricity tolerances.

The roller band device includes a chassis or carriage comprising a flatrectangular side plate 32 and a U- shaped side plate 33 which areconnected together and held in spaced parallel relation with each otherby tie rods 34 which are secured to the plates by cap screws 35. Thechassis is mounted within and secured to housing 30. In this connectionit is to be noted that side plate 32 has a pair of holes v36 sized toaccommodate cap screws 37 that are screwed into the rear wall 32 of thehousing. The idler roller 16 and the drive roller 18 engage theconvoluted S-shaped portion of band and form a roller-band cluster whichis adapted to move as a unit linearly relative to the band. Toaccomplish this, the cluster and the convoluted S-shaped portion of theband are confined between apai'r of guide or idler rollers 38 and 40journalled'for rotation about parallel axes in precision bearings 42mounted in side plates 32 and 33 of the chassis. Rollers 38 and 40 aremounted with their peripheries tangent to parallel planes spaced fromone another by a distance less than the summation of the diameters ofrollers 16 and 18 so that, because of the way the latter rolls aremounted as hereinafter described, tension on the band tends to forcerollers 16 and 18 into line contact with one another and rollers38 andthrough the intervening 40. Thus, the contact between the band androllers 38 and 40 is along parallel lines and the initial and terminalpoints of tangency between the band and rollers 16 and 18 also lie onlinear and parallel lines.

In order to provide for linear movement of the confining guide rollers38 and 40 together with idler and drive rollers 16 and 18 relative tothe band and to assure the aforementioned line contact, rollers 16 and18 are mounted for movement relative toone another and rollers 38 and40. The means for movably mounting rollers 16 and 18 is a pivotal framewhich comprises a pair of arms 44 and 46 each having one end affixed toa shaft 48 that is rotatably mounted at its ends in side plates 32 and34 so that its axis is parallel with the axes of rollers 38 and 40. Arms44 and 46 are removably clamped toshaft 48 closely adjacent side plates32 and 34 respectively, and are joined to one another near their freeends by a tie rod 50 which is secured to arms 44 and 46 by cap screws51.

To prevent skewing of the band and variation in tangency between theband and rollers, drive roller 18 is journalled in precision bearings 52(FIG. 3) mounted in arms 44 and 46 for rotation about an axis parallelwith the axes of rollers 38 and 40 and shaft 48; and idler roller 16 ismounted for limited omnidirectional movement, i.e., axially andradially, with respect to the arms and drive roller 18. For this purposeand to relieve friction due to axial thrust, the ends of idler roller 16are provided with tapered sockets 54in which are disposed thrust balls56 that engage arms 44 and 46. The arms 44 and 46 thus act as thrustpads for the balls 56 to limit the tendency of the idler roller to moveaxially.

The construction described provides minimum friction by virtue of theline contact between the guide rollers 38 and 40 and band 20. Slippag'ebetween the band and drive roller is eliminated as a result of the largeangle of wrap of the band around idler roller 16 and drive roller 18 andthe line contact effected by guide rollers 38 and 40. Any variations inthe thickness of band 20 is compensated for by the fact that arms 44 and46 can pivot and idler roller 16 can move radially to an extent limitedby the tension in band 20.

Errors due to band expansion or contraction due, for

' example, to changes in temperature and/or stretching of the band areextremely small and can be considered negligible when the roller-bandcluster as the motion converter, is employed over a limited range midwayi ployed to drive a rotary device for accurately establishing therelative position of a linearly movable component (i.e., the table 10).For this latter purpose, (as shown in the drawings) a conventionalrotary pulse generator designated 60 with an input shaft 62 is mountedfor operation by drive roller 18. Pulse generator 60 is an incremental,direction sensing digital position transducer that converts rotarymotion into digital information and may be a device as shown in U.S.Pat. Nos. 3096442, 3193744, or 3400275. Pulse generator 60 is adapted togenerate in response to rotation of its input shaft two trains of pulsesthat are 90 electrical degrees out of phase with each other with eachpulse representing a predetermined increment of the input shafts angulardisplacement and includes bidirectional logic which determines thedirection of shaft rotation by detecting which pulse train is in leadingrelation to the other. The pulse output of the pulse generator 60 iscoupled by way of a cable 65 to a resettable electronic bi-directional(i.e., backward--forward) pulse counter 66 of conventional designadapted to produce by digital techniques from the output of the pulsegenerator a visible position readout as shown at 67 which indicateschanges in position as small as 1/ 10,000 of an inch.

It is to be understood that the bi-directional logic for determining thedirection of shaft rotation may be embodied in the counter 66 ratherthan in the pulse generator. By way of example, the bi-directional logicmay be as shown in FIG. 8 of U.S. Pat. No. 3193744 or as disclosed onpages 18-36 and 18-37 of Huskey and Korn, Computer Handbook, FirstEdition, McGraw- Hill, 1962. Other bi-directional logic circuits capableof operating on the signal output of the pulse generator to provide aninput for operating a reversible pulse counter are well-known to personsskilled in the art. Various forms of bi-directional counters suitablefor use in the present invention also are known. By way of example, thecounter may be of the type described in U.S. Pat. No. 3391342 and theposition readout may be provided by an array of NIXIE tubes.

The rotatable input shaft 62 of pulse generator 60 is connected by asleeve coupling 63 to a shaft 64 which is an integral axial extension ofdrive roller 18. Coupling 63 is secured to shafts 62 and 64 so that 62will rotate at the same angular speed as drive roller 18. Since driveroller 18 is mounted on the frame comprising arms 44 and 46, the pulsegenerator is also mounted thereon. However, to prevent damage and due toshaft eccentricity, the pulse generator housing is mounted on arm 46 forlimited radial, but not rotational, motion with respect to the arm.Mounting of the pulse generator is achieved by a flexible coupling 70which comprises a hub 72 affixed to arm 46 and a hub 74 affixed to thehousing 75 of pulse generator 60, a pair of diametrically opposed pinextensions 76 formed integral with hubs 72 and 74 respectively, anintermediate rigid circular plate 78 provided with two pairs ofdiametrically opposed pin extensions 80 projecting from opposite sidesthereof, and two flexible discs 82 made of spring metal disposed onopposite sides of plate 78, with each disc secured to one pair of pinextensions 76 and one pair of pin extensions 80. The pin extensions 76are displaced ninety degrees from pin extensions 80. Since theconstruction of the coupling will permit the two hubs to undergo limitedswiveling movement but not rotate relative to one another, and since thehubs are affixed to the pulse generator housing and arm 46, the pulsegenerator is capable of moving laterally sufficient to prevent its beingdamaged by any eccentricity of roller 18. As an alternative measure, thecoupling 70 may be replaced by a coupling as shown in US. Pat. No.3,066,502. The essential thing is that the coupling allow the encoderhousing to move laterally to compensate for runout of shaft 64 whilepreventing relative rotation between the housing and shaft 64.

Operation of the device described above is as follows. Assuming that awork piece is on table and that the table is to be moved a predetermineddistance lengthwise to perform another operation on the work piece at aprecise location, the operator resets the counter to zero and then movesthe table in the desired direction lengthwise by manipulating theappropriate mechanism of the machine. As the table is moved the rollerband moves relative to the chassis carrying the roller band cluster, andsuch relative movement between the band 20 and the chassis causes driveroller 18 to rotate, which in turn causes the input shaft of pulsegenerator 60 to rotate. As its input shaft rotates, the pulse generatorsupplies count pulses to the counter which in turn instantaneouslycounts the pulses and converts the pulse count to an exact measurementof the distance that the work piece and table have moved. The measureddistance is displayed by the counter. If the operator moves the tabletoo far and has to bring it back, the reverse movement will cause thecounter to count backward. Because of the precision with which theroller-band cluster motion converter can be built and the fact thathighly precise pulse generators of the type described are available atmoderate cost the invention makes possible a precision positionmeasuring device at a cost that is small compared with that of existingsystems capable of equal accuracy and resolution.

As previously noted, the motion converter of the invention can beemployed to drive a component linearly, i.e., to move the roller-bandcluster in one direction or the other along the band 20, in which caseit functions as a rotary to linear motion converter. This is achieved asshown in FIGS. 5 and 6 by replacing pulse generator with an electricalmotor 86 so that the motors housing 88 is affixed to the flexiblecoupling and its output shaft 90 is mounted to sleeve coupling 63. Thenwith band 20 secured to one of two relative moveable (linearly)components under tension and the chassis of the roller-band clusterdevice secured to the other relatively moveable component, operation ofthe motor will cause roller 18 to rotate and, because of the engagementof rollers 16 and 18 with band 20, the roller-band cluster will travelalong the band (or vice versa) and the two components will undergorelative linear movement. Whether the cluster or band actually movesdepends on which of the two relatively moveable components is heldstationary.

Since certain changes may be made in the above apparatus withoutdeparting from the scope of the invention herein involved, it isintended that all matter contained in the above description or shown inthe accompanying drawings shall be interpreted as illustrative and notin a limiting sense.

What is claimed is:

1. A motion converting device of the type described comprising, incombination:

an idler rollerand a drive roller;

a resilient band having first and second ends;

a carriage;

a pair of guide rollers rotatably mounted to said carriage for rotationabout parallel axes, said guide rollers being disposed so that theirperipheries are tangent to parallel planes separated by a distance lessthan the summation of the diameters of said idler and drive rollers;

support means mounted to said carriage for pivotal motion about an axisparallel with the axes of said guide rollers;

said idler and drive rollers being rotatably mounted on said supportmeans between said guide rollers, said drive roller'being mounted withits axis parallel with the axes of said guide rollers and fixed withrespect to the pivotal axis of said support means, said idler rollerbeing mounted so that its axis of rotation is capable of translationalmotion relative. to said drive roller; and

said band including an intermediate portion that is convoluted about andbetween said idler and drive rollers and is engaged at one point by oneof said guide rollers and said idler roll and at another point by theother of said guide rollers and said drive roll, said band being undertension between said ends thereof so as to urge said idler roller towardsaid drive roller and so that relative move ment of said carriage andband along a line parallel to said planes and at a right angle to saididler and drive roller axes will cause rotation of said idler and driverollers in opposite directions.

2. A motion converting device as defined in claim 1 wherein said idlerroller is mounted on said support with said drive roller for rotatingsaid drive roller so as to produce relative movement of said carriageand said band.

5. A motion converting device as defined in claim 3 further includingdrive means coupled with said carriage for moving said carriage linearlyrelative to said other portions of said band and means for restrainingsaid other portions of said band against lengthwise motion.

6. A motion converting device as defined in claim 3 further includingrotary driven means mounted on said carriage and coupled with said driveroller for rotation thereby through an angle related to the lineardistance traveled by said carriage relative to said band.

7. A motion converting device as defined in claim 6 wherein said rotarydriven means include a rotary shaft coupled with said drive roller forrotation coaxially therewith and a non-rotary component mounting saidshaft for rotation and in turn mounted onsaid support means for limitedradial motion relative to the axis of said shaft. I

8. A motion converting device according to claim 1 wherein said supportmeans comprises first and second spaced members to which said idler anddrive rollers are mounted, and further wherein said idler roller ismounted to said space members by first andsecond roller bearings mountedin the opposite ends of said idler roller and engaging adjacent surfacesof said first and second spaced members respectively.

9. A motion converting device according to claim 1 wherein said carriagecomprises a pair of spaced side plates and means holding said sideplates in fixed parallel relation to each other, said guide rollersbeing rotatably mounted to said side plates, and further wherein saidsupport means is disposed between said side plates on pivot meansattached to said side plates.

A motion converting device according to claim 9 wherein said supportmeans comprises a pair of spaced parallel arms and pivot meansconnecting said arms to said side plates, said arms extending betweensaid guide rollers.

11. A motion converting device according to claim 10 wherein said pivotmeans define a pivot axis for said support means that is located betweensaid parallel planes and is displaced laterally of said guide, idler anddrive rolls.

12. A motion converting device comprising incombination:

a carriage; I g g a pair of spaced guide rollers rotatably mounted tosaid carriage so that their axes of rotation are parallel to each other;

support means pivotally mounted to said carriage, said support meanshaving a pivot axis that is parallel to the axes of rotation of saidspaced guide rollers;

a first roller and a second roller;

said first roller being rotatably mounted to said support means'so thatits axis of rotation is fixed with respect to said support means and isparallel to and spaced from said pivot axis and the axes of rotation ofsaid guide rolls;

bearing means rotatably mounting said second roller to said supportmeans so that the axis of rotation of said second roller is parallel tosaid pivot axis and the axes of rotation of said guide rollers and satfirst rollers, said bearing means being adapted to permit translationalmovement of said second roller relative to said first roller;

said guide rollers being disposed so that their peripheries are tangentto parallel planes which are separated by a distance less than thesummation of the diameters of said first and second rollers, with one ofa said guide rollers being immediately adjacent to said first roll andthe other guide roller being immediately adjacent to said second roll;and

a resilient band having first and second ends and including anintermediate portion that is convoluted about and between said first andsecond rollers and is engaged at one point by said first roller and oneof said guide rollers and at another point by said second roller and theother of said guide rollers, said ends of said band extending away fromsaid carriage and being under tension so that relative movement of saidcarriage and band will provide rotational movement of said first andsecond rollers and a relative movement of said band and said first andsecond rollers.

13. A device according to claim 12 further including holding means foranchoring said band ends and holding said band under tension, and meansfor providing relative straight-line movement of said holding means andsaid carriage.

14. A device according to claim 12 wherein the said ends of said bandare anchored at spaced points, and commencing at one of said spacedpoints said band passes between one guide roll and said first roll, thenaround said first and second rolls to between said second roll and theother guide roll, and finally out from between said second rolls andsaid other guide roller to the other of said spaced points.

1. A motion converting device of the type described comprising, incombination: an idler roller and a drive roller; a resilient band havingfirst and second ends; a carriage; a pair of guide rollers rotatablymounted to said carriage for rotation about parallel axes, said guiderollers being disposed so that their peripheries are tangent to parallelplanes separated by a distance less than the summation of the diametersof said idler and drive rollers; support means mounted to said carriagefor pivotal motion about an axis parallel with the axes of said guiderollers; said idler and drive rollers being rotatably mounted on saidsupport means between said guide rollers, said drive roller beingmounted with its axis parallel with the axes of said guide rollers andfixed with respect to the pivotal axis of said support means, said idlerroller being mounted so that its axis of rotation is capable oftranslational motion relative to said drive roller; and said bandincluding an intermediate portion that is convoluted about and betweensaid idler and drive rollers and is engaged at one point by one of saidguide rollers and said idler roll and at another point by the other ofsaid guide rollers and said drive roll, said band being under tensionbetween said ends thereof so as to urge said idler roller toward saiddrive roller and so that relative movement of said carriage and bandalong a line parallel to said planes and at a right angle to said idlerand drive roller axes will cause rotation of said idler and driverollers in opposite directions.
 2. A motion converting device as definedin claim 1 wherein said idler roller is mounted on said support meansfor limited omnidirectional motion.
 3. A motion converting device asdefined in claim 1 including means for mounting said carriage forreciprocal linear motion relative to said band.
 4. A motion convertingdevice as defined in claim 1 further including means for mounting saidcarriage and band for relative reciprocal linear movement, and drivemeans mounted to said support means and coupled with said drive rollerfor rotating said drive roller so as to produce relative movement ofsaid carriage and said band.
 5. A motion converting device as defined inclaim 3 further including drive means coupled with said carriage formoving said carriage linearly relative to said other portions of saidband and means for restraining said other portions of said band againstlengthwise motion.
 6. A motion converting device as defined in claim 3further including rotary driven means mounted on said carriage andcoupled with said drive roller for rotation thereby through an anglerelated to the linear distance traveled by said carriage relative tosaid band.
 7. A motion converting device as defined in claim 6 whereinsaid rotary driven means include a rotary shaft coupled with said driveroller for rotation coaxially therewith and a non-rotary componentmounting said shaft for rotation and in turn mounted on said supportmeans for limited radial motion relative to the axis of said shaft.
 8. Amotion converting device according to claim 1 wherein said support meanscomprises first and second spaced members to which said idler and driverollers are mounted, and further wherein said idler roller is mounted tosaid space members by first and second roller bearings mounted in theopposite ends of said idler roller and engaging adjacent surfaces ofsaid first and second spaced members respectively.
 9. A motionconverting device according to claim 1 wherein said carriage comprises apair of spaced side plates and means holding said side plates in fixedparallel relation to each other, said guide rollers being rotatablymounted to said side plates, and further wherein said support means isdisposed between said side plates on pivot means attached to said sideplates.
 10. A motion converting device according to claim 9 wherein saidsupport means comprises a pair of spaced parallel arms and pivot meansconnecting said arms to said side plates, said arms extending betweensaid guide rollers.
 11. A motion converting device according to claim 10wherein said pivot means define a pivot axis for said support means thatis located between said parallel planes and is displaced laterally ofsaid guide, idler and drive rolls.
 12. A motion converting devicecomprising in combination: a carriage; a pair of spaced guide rollersrotatably mountEd to said carriage so that their axes of rotation areparallel to each other; support means pivotally mounted to saidcarriage, said support means having a pivot axis that is parallel to theaxes of rotation of said spaced guide rollers; a first roller and asecond roller; said first roller being rotatably mounted to said supportmeans so that its axis of rotation is fixed with respect to said supportmeans and is parallel to and spaced from said pivot axis and the axes ofrotation of said guide rolls; bearing means rotatably mounting saidsecond roller to said support means so that the axis of rotation of saidsecond roller is parallel to said pivot axis and the axes of rotation ofsaid guide rollers and said first rollers, said bearing means beingadapted to permit translational movement of said second roller relativeto said first roller; said guide rollers being disposed so that theirperipheries are tangent to parallel planes which are separated by adistance less than the summation of the diameters of said first andsecond rollers, with one of said guide rollers being immediatelyadjacent to said first roll and the other guide roller being immediatelyadjacent to said second roll; and a resilient band having first andsecond ends and including an intermediate portion that is convolutedabout and between said first and second rollers and is engaged at onepoint by said first roller and one of said guide rollers and at anotherpoint by said second roller and the other of said guide rollers, saidends of said band extending away from said carriage and being undertension so that relative movement of said carriage and band will providerotational movement of said first and second rollers and a relativemovement of said band and said first and second rollers.
 13. A deviceaccording to claim 12 further including holding means for anchoring saidband ends and holding said band under tension, and means for providingrelative straight-line movement of said holding means and said carriage.14. A device according to claim 12 wherein the said ends of said bandare anchored at spaced points, and commencing at one of said spacedpoints said band passes between one guide roll and said first roll, thenaround said first and second rolls to between said second roll and theother guide roll, and finally out from between said second rolls andsaid other guide roller to the other of said spaced points.